Human hepatitis B virus (HBV), a member of the hepadnavirus family, causes acute and chronic hepatitis and is strongly associated with hepatocellular carcinoma. though a vaccine exists to prevent acquisition of infection, no therapy exists for the 250 million individuals worldwide who are already chronically infected. Rational development of antiviral drugs for such cases will require detailed knowledge of the mechanisms of viral replication. The purpose of this project is to deepen our fundamental understanding of the mechanics of hepadnaviral replication. Hepadnaviruses replicate via reverse transcription of an RNA intermediate. In this application studies are proposed to elucidate each of the major steps in viral genomic replication, including (i) the biogenesis of the RNA template; (ii) its encapsidation into subviral core particles and (iii) the reverse transcription reactions that generate first (-) and then (+) strand DNA, with emphasis on the initiation steps in these reactions. this will be done both through the study of mutant hepadnaviral genomes in cell culture and through the establishment of in vitro systems designed to reconstruct individual steps in replication. Using similar approaches, additional studies will examine how the products of reverse transcription are transported from cytoplasm to nucleus in the establishment and maintenance of infection, and how HBV replication is inhibited by coinfection with hepatitis delta virus, another important human pathogen.